Secondary Organic Aerosol Formation from Aqueous Ethylamine Oxidation Mediated by Particulate Nitrate Photolysis

Atmospheric ethylamine (EA) is emitted by various primary sources and can be found abundantly in the gas and particle phases. Nitrate (NO₃^–) is one of the most abundant inorganic compounds and has been found to coexist with amines in ambient particles. The photolysis of NO₃^– can produce oxidants such as the OH radical, NO₂, O(³P), and N(III), which can lead to the decay of particulate EA. Moreover, the degradation of EA forms carbonyl species, which are precursors to brown carbon (BrC) formation. In this study, we investigated the aging of EA-containing particles mediated by NO₃^– photolysis under different relative humidity (RH) and initial pH conditions under 300 nm UV irradiation. The more acidic (pH 0.0, 0.2, and 0.6) particles (EA:H⁺ molar ratio = 4:4.25, 4:4.5, 4:5 at 70% RH) exhibited an increase in pH, while the less acidic (pH 5.0, 4.8, 4.7, and 5.1) particles (EA:H⁺ = 4:4 at 40%, 55%, 70%, and 85% RH) showed a decrease in pH as a result of photooxidation. We attributed these contrary pH changes to the combination of the HONO evaporation, which increases the pH, and the EA reactions, which decrease the pH. The decay rates of NO₃^– and EA appear not to be sensitive to RH and pH within experimental uncertainties. We proposed EA reaction pathways in the presence of oxidants produced from NO₃^– photolysis based on product speciation. We also observed the formation of water-soluble organics (BrC and an organic phase) as a potential secondary organic aerosol (SOA). This study sheds light on the particulate sink of EA and its potential in BrC and SOA formation mediated by NO₃^– photolysis in the atmosphere, providing new insights into the aging of amines in atmospheric aerosols.

This study shows particulate ethylamine decay during nitrate photolysis could form water-soluble secondary organics (BrC and an organic phase), providing insight into the atmospheric amine sink and aging.